Magnetorheological fluids that comprise suspensions of magnetic particles such as iron or iron alloys in a fluid medium are well known. The flow characteristics of these fluids can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field due to suspension of the particles. The ferromagnetic particles remain suspended under the influence of magnetic fields and applied forces. Such magnetorheological fluids have been found to have desirable electro-magnetomechanical interactive properties for advantageous use in a variety of magnetorheological (MR) damping devices, such as rotary devices including brakes and clutches, and linear-acting devices for damping linear motion or for providing controllable dissipative forces along the damper's axis.
High performance controlled damping applications, such as those used in passenger vehicle suspension systems, preferably provide a relatively low damping force at low speeds for comfort, and provide relatively high damping force at higher speeds for safe handling of the vehicle. Thus, continuously variable real-time damping (CV-RTD) actuators have become increasingly popular. The damping performance of a MR fluid based CV-RTD is largely dependent on the force-velocity characteristics of the damper. FIG. 1 illustrates the optimum force-velocity characteristics of a damper used in automotive applications. The slope of the off-state force-velocity curve should be as low as possible for a smooth ride, with a desirable value of approximately 600 N-s/m. The on-state force-velocity curve preferably has an initial slope in the range of 5-30 kN-s/m up to a velocity of 0.1 to 0.4 m/s and a final slope similar to that in the off-state. The desirable maximum on-force should be limited to a suitable value (e.g., 4500 N) at 2 m/s. The ratio of the damping force when the damper is in the on-state (on-force) to the damping force when the damper is in the off-state (off-force) at a given velocity is known as the turn-up ratio. It is desirable to have a turn-up ratio of at least 3 to 6 at a velocity of 1 m/s for good control of the vehicle chassis dynamics.
FIG. 2 shows a known monotube MR damper 10 having a piston 12 sliding within a hollow tube 14 filled with MR fluid. The piston 12 is attached to a hollow rod 18 that slides within a sealed bearing 20 at one end of the body of the damper 10. The piston 12 contains a coil 22 carrying a variable current, thus generating a variable magnetic field across a flow gap 24 between an inner core 26 and an outer shell or flux ring 28 of the piston 12. A bearing 30 having relatively low friction is disposed between the flux ring 28 and the tube 14. The flux ring 28 and the inner core 26 of the piston 12 are held in place by spoked end plates 32. Terminals 34 of the coil 22 extend through the hollow rod 18 and are provided with suitable insulation for connection to a source of electricity. One end portion 36 of the tube 14 is filled with inert gas which is separated from the MR fluid by a floating piston 38. The floating piston 38 and inert gas accommodate the varying rod volume during movement of the piston. U.S. Pat. No. 5,277,281 discloses a similar MR damper.
FIG. 3 illustrates the force-velocity characteristics of the type of MR damper disclosed in FIG. 2. Clearly, in comparison to the preferred curves of FIG. 1, improvements in the force-velocity characteristics of conventional MR dampers are desirable. Although the above-described conventional MR dampers may perform adequately is certain applications, these devices do not achieve the required turn-up ratio and substantially stiction free performance near zero velocity for realistic automotive applications. Conventional monotube dampers do not provide sufficient tuning capability to effectively control the damping characteristics as represented, for example, by the slope of the force-velocity curves. Also, conventional dampers have an unnecessarily long length for a given performance.
Therefore, there is a need for a more compact MR damper capable of more effectively and controllably damping motion.